Nonconcordance of mutagenicity and carcinogenicity assays results when data from short-term mutation assays do not predict the results of 2 year bioassays. Our studies on those chemicals which produced positive mutagenicity in vitro and failed to produce carcinogenicity in NTP bioassays demonstrated the requirement for cell proliferation in the early stages of chemical exposure for positive carcinogenicity results. Chemicals which fail to cause cell proliferation also fail to cause carcinogenesis, regardless of their activity in mutagenesis assays. We have examined a number of chemicals to date and observed that the ability to cause cell proliferation and carcinogenesis is organ- specific and site-specific within an organ, and may be sex- and species-specific. We are also studying the mechanism(s) whereby chemicals that induce peroxisomes or cytochrome P450 isozymes produce hepatocarcinogenesis, and the relationship of this effect to human risk following exposure to these chemicals. Recent studies have used transgenic (Big Blue) mice to detect in vivo mutagenesis induced by diaminotoluene isomers. Oxazepam, a benzodiazepine compound, is a central nervous system depressant which is widely prescribed for the treatment of anxiety. In NTP rodent bioassays, exposure of male B6C3F1 mice to 0, 125, 2,500 and 5,000 ppm of oxazepam resulted in hepatocellular carcinoma in 47, 38, 100 and 100%, respectively, of the mice. Male B6C3F1 Big Blue transgenic mice were fed 2500 ppm oxazepam or control diet alone for 180 days and sacrificed on the next day. The mutant frequency of lacI in control mice was 5.02 x10-5 whereas the MF in the oxazepam-treated mice was 9.17 x10-5, a significant increase (p<0.05). Correction of the mutant frequency of lacI from the oxazepam- treated mice for clonality resulted in a decrease in the mean mutant frequency to 8.15 x10-5. This study demonstrates that the in vivo Big Blue transgenic rodent mutation assay can detect mutations derived from a carcinogen that did not induce gene mutations in vitro. Moreover, the sequencing of the recovered mutants can distinguish between the mutation spectrum from treated mice compared to that from control mice, thereby confirming the genotoxic consequences. - mutagenicity, carcinogenicity, bioasays, carcinogenesis, peroxisomes, P450 isozymes